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The steady state analysis allows for the input of multiple sinusoidal forces, to applied in each of the six degrees of freedom and applied to any node within the model. Each sinusoidal force is defined by the frequency, phase and amplitude. Each sinusoidal force can be applied to a single or to multiple nodes. As for the Force profile and Base Acceleration analyses, the Steady State analysis will return results for specific nodes, or for a specific time in the analysis in the form of a snapshot, or in the form of minimum and/or maximum nodal displacements and member end forces. Snap shots can then be added to static loadcases within Masterframe, to account for the dynamic effect of the input loading.
On first selecting the Steady State tab, the screen layout is as shown below.
The input sinusoidal forces are defined in the table at the bottom left hand side of the screen. The results are is contained in the right hand pane. Forces are defined by the frequency and phase along with the force direction and the nodes to which the forces are applied. When a sinusoidal force is input, the graph to the right of the force definition will display the force profile in the selected force direction.
An example of a sinusoidal force of frequency 10 Hz and a phase of 0 radians, with an amplitude of 15kN in the x-axis direction and applied at three nodes in the FE surface, is shown below.
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By defining multiple time varying loads in this way it is possible to built up complicated time dependant loadings and apply them to a range of nodes within a model.
To apply loads to nodes, the nodes can be select either by typing in the node number, or by using the mouse and selecting the individual nodes from the frame graphical display. The loads associated with the selected line in the force input table will be displayed graphically at the selected node.
Analysing the structure
Once the required system of loading is input, the analysis settings can be defined. The analysis is carried out in time steps over the required time period. Both the time step or interval and time period can be entered in the appropriate input boxes. The damping ratio, expressed as the percentage of the critical damping. Thus an input of 1.0 represents critical damping while an input of 0.03 represents damping of 3%.
The magnification input controls the scaling of the animation as the analysis is run.
In the Steady State analysis, there are a number of ways to run an analysis. To run the analysis using all the input loads, click on the .png){kind=small}
icon. It is also possible to run the analysis for a single input sinusoidal loading,. To do this, the required line in the input table needs to be highlighted, then the .png){kind=small}
icon can be clicked and the analysis is then run using only the forces in the highlighted row of the table.
Since the analysis is carried out in time steps, the number of calculations it therefore related to the total time and the number of intervals. Thus a smaller time step will give a better resolution in the results, but will also increase the number of calculations and so increase the time required to complete the analysis. When selecting the time steps, it is also worth considering the effect periodic motions may have - if the time steps and a resulting periodic displacement of the structure coincide, the time step may not fully capture the effects of the load. It is, therefore, advisable to adjust the time steps to ensure that the results adequately capture the structural response.
Once an analysis has been run, the results will be displayed in the right hand pane. A typical results layout is shown below. Results are given for the nodal displacements, velocity at the nodes and the acceleration at the nodes. The results can be displayed for specific nodes, for a particular snapshot in time, or for the max/min displacements and forces in members.
Note: Analysis results are only displayed for the selected results outputs. Therefore, it is necessary to select the desired result types first, then run the analysis. If no results are selected, the analysis will run, but no results will be displayed. For an initial analysis run, the Min/Max displacements can be helpful, since this will identify the nodes with the largest positive or negative displacement and the members with the largest (positive or negative) forces. This can therefore assist in selecting nodes to use with the Node Watches outputs or identifying times for use with the creation of Snapshots.
Node watches give displacement, velocity and acceleration results for specified nodes, with results given in x, y and z axes directions. The software will give the results for a selected node at the time intervals specified in the Time Step. The software will also produce a graph of the outputs for each of the selected displacement, velocity or acceleration.
To add a node to the Node Watch list, click on .png){kind=small}
. This will open a pop-up window, shown below. The node number is input either by typing in the node number. Only a single node can be input at a time. The axis the results are to be shown in is select using the radio buttons.
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Once nodes are input, running the Force Profile analysis will then display the results for each of the selected nodes. The node to display the results for can be selected using the drop down.
With a node selected, the results for the displacement, velocity and acceleration will be displayed at each time step within the input total time. The graph will also display the selected result over the total input time. Using the mouse wheel, the x-axis scale of the graph can be changed to modify the displayed graph output. Using the mouse pointer, hovering over a point in the graph will display the value of the result.
A typical results display for a Node Watch is shown below.
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A snapshot represents the results for the full model at a specific point in time. A snap shot represents the nodal displacements and member forces captured at a specific point in time. Snap shots can be created for the time intervals used, where the displacements and forces in the model are explicit calculated. Since for times between the steps the model forces and displacements are not calculated, if a snap shot is created for a time which is not a multiple of the time step, no forces or displacements will be reported.
To add a snap shot at a point in time, enter the time in the input box. The time is to be added in seconds. Once done, click on the icon. The added snapshot will now be available from the drop down list. To get the results for an added snapshot, the analysis must be run after adding the snap shot.
Once the analysis is completed the results for all the added snapshots will be available for viewing. To view the results for a snapshot, select the required snapshot from the drop down list and select the required results using the Nodal Displacements or Member Forces radio buttons.
Snapshots can also be created automatically in the Min/Max Displacements Member Forces results area. This creates a snap shot for each of the maximum or minimum displacements and member force results. The use of this function is covered in the next section.
The minimum and maximum displacements and member forces results area reports the largest positive and negative nodal displacements in each of the six global axes degrees of freedom and the largest positive and negative member end forces for the axial, torsion shear and bending in the major and minor member axes. Within the Min/Max Displacements Member Forces area, results can be calculated and displayed for nodal displacements of all or a selection of nodes in the model and/or for member end forces for the whole model or a user defined selection of members. Snapshots can also be created for the nodal displacements and/or member end forces.
On first entering the Min/Max Displacements Member Forces area, the screen layout is as shown below. No results are selected, with both the disparagement and member forces area being greyed out.
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To select the nodal displacement and member end forces, check the appropriate checkbox. Once a checkbox is selected, the relevant area becomes active. The "All" or "Selection" options are selected using the radio buttons. When the "Selection" option is selected, clicking on .png){kind=small}
opens an additional pop-up window. Here, nodes can be selected to give the appropriate results for. Nodes are selected graphically from the graphics pane. Multiple selection of nodes can be done by windowing around nodes using the mouse cursor. If Node Groups have been created in Masterframe, then the node groups can be selected from the Node Group drop down.
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Once the selected results and nodes have been, made, the analysis can be run using the icon. Once complete, the results area will be completed with the required results. A typical results screen is shown below.
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When snap shots have been created, either directly in the Snapshot results or as part of the Min/Max displacement and member forces results, these can be added to a loadcase within Masterframe, such that the effects of the Force Profile loading can be included within a load case. Snapshots can be added to a Masterframe load case by going to Loads>Dynamic and Seismic Loads>Dynamic Snap Shot Loads. This opens a pane on the right hand side of the screen. The required load case is selected from the drop down at the top of this pane. To attach a snap shot, check the "Attach Dynamic Snapshot" checkbox. The snap shot to be added to a loadcase is selected from the "Select a force snapshot" drop down. Snap shots are labelled with the time stamp. Therefore to add a particular snap shot, the time of the appropriate snap snot needs to be noted in the Dynamic results area. The factor to be applied to the snap shot is entered in the "Apply a factor" input.